nullable?

# File actionpack/lib/action_dispatch/journey/gtg/builder.rb, line 66
        def nullable?(node)
          case node
          when Nodes::Group
            true
          when Nodes::Star
            # the default star regex is /(.+)/ which is NOT nullable but since different
            # constraints can be provided we must actually check if this is the case or not.
            node.regexp.match?("")
          when Nodes::Or
            node.children.any? { |c| nullable?(c) }
          when Nodes::Cat
            nullable?(node.left) && nullable?(node.right)
          when Nodes::Terminal
            !node.left
          when Nodes::Unary
            nullable?(node.left)
          else
            raise ArgumentError, "unknown nullable: %s" % node.class.name
          end
        end